1. Field of the Invention
The present invention relates generally to locating emitters from a moving platform using RF interferometers. More specifically, it relates to accurate emitter ranging using a long baseline interferometer (LBI) and an initial angle of arrival (AOA) measurement.
2. Description of the Related Art
Several techniques have been developed using long baseline interferometers (LBI) to perform emitter ranging, as opposed to emitter geolocation or precision direction finding (DF) alone. As discussed in A. L. Haywood, "Passive Ranging by Phase-Rate Techniques" (Wright-Patterson AFB Tech. Report ASD-TR-70-46, December 1970), accurate ranging requires only the precise measurement of emitter bearing rate-of-change. This means that differential, not absolute, resolution of the long interferometer baseline is required, as described in Kaplan, U.S. Pat. No. 4,734,702.
A typical radio frequency interferometer system includes pairs of antennas in which an emitter is located by utilizing the phase difference of the signal arriving at two different antennas. However, the phase measurements of the interferometer can be emitter signal angle-of-arrival (AOA) ambiguous if the baseline of the antenna array, that is the separation of the two antennas used to measure signal phase, is greater than half the wavelength of the incoming signal. The number of ambiguous AOA's is closely approximated by the ratio of the interferometer baseline to the emitter wavelength and one key to successful emitter range estimation is to correctly resolve this ambiguity. A gross error is said to occur if the ambiguity resolution is done incorrectly.
However, as the interferometer baseline length increases, therefore increasing the number of AOA ambiguities, the phase measurement accuracy increases, and the rapidity of emitter ranging to the required accuracy improves due to the more accurate bearing measurements. Thus, the desire for accurate bearing measurements, requiring a long interferometer baseline, conflicts with the need for robust phase or AOA ambiguity resolution, which is easier to accomplish with a short baseline.
Although only two antenna elements are required to measure signal phase, conventional interferometer designs utilize additional antenna elements to control the gross error rate of a long baseline interferometer or LBI resulting from the inherent AOA ambiguity in phase measurement. One approach to doing this is disclosed in U.S. Pat. No. 4,638,320 by Eggert et al. Conventional techniques also require the system to include phase calibration to assure adequate phase tracking over the operational field of view (FOV) and bandwidth, and place significant constraints on frequency measurement accuracy. These make conventional systems designed for precision emitter ranging using RF interferometers both heavy and expensive, and therefore of limited utility.
One technique to overcome this complexity and weight is to use an AOA ambiguous long baseline interferometer or LBI. One approach to passive ranging utilizing an LBI which may have only two antenna elements is disclosed in Pentheroudakis (U.S. Pat. No. 3,935,574) which is incorporated herein by reference. This approach resolves the AOA ambiguities, which are identical to the antenna array grating lobes, by phase tracking the emitter signal during the relative motion of the platform containing the antenna array. "Lobe tracking" is utilized in which (2n) solutions are set up for an array with a length of n(.lambda.), where .lambda. is the emitter signal wavelength and n is an integer. Only one of the potential solutions converges to the true solution with the remainder being rejected as diverging. Phase tracking is continuously performed by a lobe tracking process in order to eliminate all but one of the potential solutions so that the ambiguity integer m, which determines the number of 2.pi. cycles which must be added to the interferometer phase measurement to correctly identify the correct AOA, is determined.
The phase tracking approach has certain drawbacks. For instance, the signal can be interrupted by terrain blockage or intermittent emitter operation and the trend on the ambiguity integer m (phase tracking) is lost. Additionally, vibration can distort the trend and hinder correct ambiguity resolution. Observer attitude motion can cause large changes in the ambiguity integer, m. These changes are difficult to separate from translational motion relative to the emitter.
Another drawback to this phase tracking method is that the number of lobes, i.e. potential solutions, is determined by the ratio of d/.lambda., where d is the baseline length and .lambda. is the signal wavelength. At 18 GHz, with .lambda.=0.66", if d=400", there are more than 600 (d/.lambda.) potential solutions.
Another passive ranging approach utilizes a short baseline/long baseline interferometer or SBI/LBI system in which at least two SBI measurements separated in time are needed to resolve the two-antenna element LBI ambiguity. This approach is described in "Passive Ranging Method and Apparatus", (U.S. Pat. No. 4,734,702), by Kaplan, which is incorporated herein by reference and will be referred to subsequently as conventional SBI/LBI.
U.S. Patent No. 4,734,702 discloses two approaches utilizing SBI/LBI. One approach locates the target with SBI derived measurements and uses the SBI range to predict the LBI phase change. This approach requires some SBI location convergence before improving it with the LBI. The use of the SBI phase difference to initially compute a slant range means this method will not initially converge faster than a more conventional SBI-only system until a range accuracy sufficient to resolve the LBI has been achieved. In order to overcome this slow initial convergence to the range estimate, a second technique is used instead, if the slow initial convergence is intolerable.
This second technique utilizes the SBI unit direction-of-arrival vector (DOA) u to predict the LBI phase change. This technique does not require location to any accuracy before differentially resolving the LBI with sequential SBI measurements, and hence provides rapid convergence to an accurate range estimate. However, this SBI/LBI technique has the drawback of limiting the SBI/LBI baseline ratio, and requiring the use of a medium baseline interferometer (MBI) in many cases. It also requires a two dimensional (2-D) SBI to measure emitter direction of arrival or DOA, as opposed to just a one dimensional interferometer array measuring AOA.
The 2-D SBI is required because utilizing the change in SBI phase to predict the change in LBI phase requires generating the following DOA unit vector: EQU u.sub.SBI =cos (e) cos (a)i+cos (e) sin (a)j-sin (e)k (1)
where
e=elevation angle measured by SBI, PA1 a=azimuth angle measured by SBI, and PA1 i,j,k correspond to x,y,z in FIG. 1. PA1 d.sub.LBI.sbsb.1, d.sub.LBI.sbsb.2 =LBI baseline vectors at t.sub.1, and t.sub.2, respectively.
The unit vector, u, is required to predict the LBI phase change, .DELTA..phi..sup.p.sub.LBI at two different times t.sub.1 and t.sub.2 via the relation: where, ##EQU1## u.sub.SBI.sbsb.1, u.sub.SBI.sbsb.2 =unit SBI vectors at t.sub.1, and t.sub.2, respectively.
The DOA unit vector, as opposed to just the AOA angle, is thus required because the LBI baseline changes its orientation in space with time.
The MBI is required because, in using the SBI phase measurements to predict the LBI differential phase, SBI non-constant errors, e.g. system thermal noise, are scaled up in the ratio of LBI to SBI baselines. If the SBI RMS phase errors are on the order of ten electrical degrees, the baseline ratio cannot be greater than 8:1 before another antenna is required. Therefore, if the desired range estimation performance requires an ultimate SBI-to-LBI baseline ratio of more than 8:1, an intermediate LBI, referred to as an MBI above, must be used. Thus, two or more additional antenna elements are required.
This baseline restriction on the SBI in relation to the LBI length arises in the SBI/LBI phase ambiguity resolution approach utilizing multiple SBI measurements as follows.
FIG. 2 shows a platform 20 moving between two points. At time, t.sub.1 and t.sub.2, SBI and LBI measurements are made from signals coming from stationary emitter 10. The angle .DELTA.a or azimuth bearing spread at the emitter is formed by the movement of the platform 20 at speed v relative to the emitter. For the purpose of this simple example, the range r, from the emitter 10 to platform 20 at time t.sub.2 can be found from .DELTA.a by: ##EQU2## In practice, determining r represents a rather elaborate estimation problem. Equation 2 becomes, for this simple example: ##EQU3## where, d.sub.LBI =baseline length EQU .phi..sup.m.sub.LBI.sbsb.2 -.phi..sup.m.sub.LBI.sbsb.1 =.DELTA..phi..sup.m.sub.LBI is the measured phase difference between time t.sub.2 and t.sub.1,
which is related to the unknown bearing spread by the approximation: ##EQU4## where .DELTA.n=n.sub.2 -n.sub.1. .DELTA.a can be found from equation 5 and substituted into equation 3 to get range, once the ambiguity integer .DELTA.n is determined. This "phase resolution" is carried out using the SBI phase measurements by solving for ##EQU5## to SBI accuracy. This is not accurate enough to give a solution for .DELTA.a providing rapid emitter location, but it is accurate enough to predict the unambiguous LBI phase by: ##EQU6## Equation 7 is used to find An by adding 2.pi. to the measured phase until the inequality ##EQU7## is satisfied.
Note that the SBI errors are scaled up by a factor proportional to the ratio of the LBI to SBI baseline lengths in equation 7. This phase prediction error, as well as other errors due to antenna vibration-induced motion and inertial navigation system or INS inaccuracies, should have a numerical magnitude consistently less than .pi./2 radians. This crucial requirement limits the LBI-to-SBI baseline ratio ##EQU8## which imposes undesired restraints on prior art systems.
This limitation on either how fast the range estimate converges or on the SBI/LBI baseline ratio, has proven to be a significant drawback in practical applications, since it severely restricts placement of LBI antennas. Current SBI/LBI installations tend to be awkward, require costly installation, and do not have the baseline length desirable for very rapid ranging.
Table 1 summarizes the features of the two conventional SBI/LBI techniques.
TABLE 1 ______________________________________ Technique Elements in Conventional SBI/LBI 1 2 ______________________________________ Sequential SBI Phase Measurements Yes Yes SBI Range Required Yes No Unit DOA vector generated No Yes sequentially from SBI phase measurements ______________________________________
Differential phase resolution is utilized in both of these SBI/LBI approaches, so that the LBI baseline does not need to be calibrated, but the conventional schemes are very sensitive to SBI accuracy. It is desirable to have a system that is not limited by this sensitivity.
The baseline restriction existing in the conventional SBI/LBI approach necessitates the addition of more antenna elements to obtain the LBI baseline required to achieve the desired range accuracy quickly. However, the placement of the antenna elements is also restricted by the baseline constraint. This is a serious practical constraint on many air platforms. On tactical aircraft in particular, the placement of additional antenna elements are subject to airframe available space, which rarely coincides with the current SBI/LBI design limitations.
In both conventional SBI/LBI approaches, the SBI phase accuracy is critical because of the scaling of the SBI phase up to LBI to resolve the LBI phase ambiguity, as shown above, or to obtain an initial estimate of emitter range and thus predict the LBI phase needed to resolve the integer ambiguity, as also described in U.S. Pat. No. 4,734,702. Both approaches require sequential SBI measurements to be utilized in conjunction with sequential LBI phase measurements.